Pink hydrangea turns green in heat

I read your article about green hydrangeas, and it didn't quite answer my question. We have a light pink hydrangea, and as soon as the weather turns hot (we live in Modesto, CA) the pink flowers turn green. It happens almost overnight. Is there anything we can do to prevent this?

One question comes to mind -- how long have you had this plant? Has it always had this color pattern? There are many hydrangea varieties, with a number of different color variations, so is it possible that it's natural for yours to go from pink to green? Other than that, there are a couple of suggestions for preventing flowers turning green -- you can try increasing the amount of water; also, if there's some way to provide some shade from the hottest part of the day. Don't know if any of this is practical, or if it would work, but I've seen these tips mentioned.